The present invention relates to the formation of impermeable electroplated metal structures.
The process of forming free-standing articles by means of electroplating is well known. Typically, an electroplate is deposited on a conductive mandrel and subsequently removed therefrom. For example, a cylinder of conductive wax may be covered with an electrodeposit, and then the wax may be melted away to leave a free-standing replica of that cylinder. Electroforming has been used to make duplicating plates, thin-walled sections, precision parts, and parts difficult or impossible to make otherwise. Among the products made are wave guides, pitot tubes, molds and dies for thermoplastics, fine mesh screens, and phonograph record masters.
While many useful objects are made by electroforming, the performance of any particular object will be limited by the properties of the electrodeposit. These properties in turn will be dependent upon the material composition, deposition parameters and any post-deposition processing. A general characteristic of electrodeposits of materials such as nickel, copper and other common metals, is that they are comprised of essentially columnar-type structures. In other electrodeposits a certain degree of crazing or microcracking is present. Both these factors may be found to produce some permeability to gases in electroformed objects.
Electroformed objects would desirably be used as containers in which metal powders may be hot isostatically pressed (HIP). These are often complexly shaped as reflections of the objects being fabricated. Commonly, they are made from nickel or iron base sheet metal of about 1-6 mm thick. After a container is fabricated it is filled with metal powders, evacuated to 10.sup.-1 torr or better and sealed. Then temperature and pressure are applied to the exterior of the container in order to collapse the container and thereby densify the powder contained within. For superalloy powders the parameters would typically be about 100 MPa at 1200.degree. C. for two hours; the container having been evacuated to about 10.sup.-1 torr. After removal from the HIP unit and cooling, the container which is generally diffusion bonded to the powder is usually removed by machining or chemical milling.
Thus, it would seem that it would be a natural application of electroforming to make the expendable HIP containers. But due to the permeability of electroformed containers of materials such as nickel, an adequate vacuum can not be achieved and sustained within the container during the HIP operation.
The invention herein will be seen to relate to differential fusing of layers in electroforms. Melting of thin metal films is commonly encountered in brazing, soldering and the like, wherein a thin layer of meltable material is interposed between two surfaces. Electroplating may be used to place such layers. For example, Wells, U.S. Pat. Nos. 3,675,311 and 3,708,866 disclose a method wherein a thin film of titanium or niobium is used to join nickel. In Woodward, U.S. Pat. No. 3,854,194 a titanium diffusion bonding process is disclosed wherein sequential electroplate layers of nickel, copper, and silver are applied to the surface to be bonded. Of course, the objects of bonding or brazing are quite dissimilar from those involved in forming impermeable structures and HIP containers. In the aforementioned patents and similar joining art, total melting is inherent in the process and electroplating is merely a convenient method of preplacing the layer.
There are also fusible coatings which may be applied to objects by thermal or plasma spraying. Typically, these are hardfacing-type materials; after deposition the coatings are heated by torch or furnace above thin melting points to cause melting and coalescence of the coatings. As deposited sprayed coatings are obviously porous (in contrast to the apparent density of electroplates). Therefore, the object of fusing sprayed coatings is to increase their density, adherence and general properties. While free-standing objects may be made by spraying coatings onto removable mandrels, it should be apparent that if the sprayed object is of a fusible material, it will collapse during the fusion operation.